How do I do the voice over on the slides?

You can record using your computer's audio. You can record the audio using digital device and upload as a library resource. Then, drag and drop the audio file in as a layer with slides. Adjust your audio first, and then make your slides match the timing of your audio. I use a simple, cheap Logitech USB mic to record directly into Camtasia. Camtasia creates an audio .wav file. I do this for each slide separately so that I can make adjustments to slides later. Some record the whole audio into Camtasia and then edit the audio after.